The outcomes of related measures in Colorado and Oregon were uncertain.
The measure sets up a system of state-licensed marijuana growers, processors and retail stores, where adults over 21 can buy up to an ounce. It also establishes a standard blood test limit for driving under the influence.
Legalization could help bring in hundreds of millions of dollars a year in pot taxes, reduce small-time pot-related arrests and give supporters a chance to show whether decriminalization is a viable strategy in the war on drugs.
The sales won't start until state officials make rules to govern the legal weed industry.
Promoted by New Approach Washington, I-502 calls for a 25 percent excise tax at each stage from the growers on until it is sold in stores to adults 21 and over.
They could buy up to an ounce of dried marijuana; one pound of marijuana-infused product in solid form, such as brownies; or 72 ounces of marijuana-infused liquids.
The cannabis would be subject to testing to establish its THC content, and labeled accordingly.
State financial experts estimate it could raise nearly $2 billion in tax revenue over the next five years, with the money going toward education, health care, substance abuse prevention and basic government services.
When state and federal laws conflict, federal law takes precedence. Federal authorities could sue in an attempt to block I-502 from taking effect. The Justice Department has given no hints about its plans.
The campaign was notable for its sponsors and supporters, who ranged from public health experts to two of Justice's top former officials in Seattle, U.S. Attorneys John McKay and Kate Pflaumer.
The effort raised more than $6 million in contributions, with more than $2 million of that coming from Progressive Insurance Co. founder Peter Lewis, who used marijuana to treat pain from a leg amputation.
The ample fundraising allowed New Approach Washington to run television ads through the campaign's final weeks.
Meanwhile, I-502 had little organized opposition. Some in law enforcement and public health are concerned that increased access will lead to increased abuse, especially among teens.
Others who opposed the measure did so because it didn't go far enough, and that the blood test limits were arbitrary and could affect medical marijuana patients. Still others worried about a possible federal-state law clash.
For many voters, it came down to the notion that decades of marijuana prohibition have done more harm than good.
"It's ridiculous to be trying to maintain the law enforcement effort -- all the people, all that money, all those resources -- to prosecute marijuana use," supporter Karla Oman said. "Tax it, legalize it, everybody wins."
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